Density measuring instrument having electromagnetic suspension with a variable spring constant

ABSTRACT

An electric coil is provided for the purpose of carrying alternating current. An electrically conducting sphere placed within the magnetic field of the coil incurs eddy currents resulting in magnetic repulsion between the sphere and the coil which is maintained at a null position by means of an autoregulation or feedback circuit. Voltage or current applied to the coil to maintain the sphere at a null position is an indication of the buoyant forces acting upon the sphere and thus is representative of the density of the gas or liquid surrounding the sphere. The coil and sphere are enclosed in a magnetic enclosure to provide a flux path and the top plate of the enclosure is provided with an opening of a size selected to alter the magnetic field of the coil and thereby the equivalent spring constant of the system.

ilnited Etates Patent 9] Fiet [451 Apr. re, 1973 [75]' Inventor: OwenOrlando Fiet,

Beach, Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

[22] Filed: Mar. 29, 1971 2] Appl. No.: 129,030

Redondo 52 use]. ..73/3o,73/453,73/32 51 rm. (:1. ..G01n9/20 5sFieldofsearch ..73/19,30,453,32,

Primary Examiner-Richard C. Queisser Assistant Examiner-C. E. Snee, III

Attorney-Daniel T. Anderson, William B. Leach and Donald W. Graves [5 7]ABSTRACT An electric coil is provided for the purpose of carryingalternating current. An electrically conducting sphere placed within themagnetic field of the coil incurs eddy currents resulting in magneticrepulsion between the sphere and the coil which is maintained at a nullposition by means of an autoregulation or feedback circuit. Voltage orcurrent applied to the coil to maintain the sphere at a null position isan indication of the buoyant forces acting upon the sphere and thus isrepresentative of the density of the gas or liquid surrounding thesphere. The coil and sphere are enclosed in a magnetic enclosure toprovide a flux path and the top plate of the enclosure is provided withan opening of a size selected to alter the magnetic field of the coiland thereby the equivalent spring constant of the 7 Claims, 4 DrawingFigures [56] References Cited UNITED STATES PATENTS 3,581,556 6/1971Salvinski et al ..73/453 2,524,600 10/1950 Raymond et al. ..73/453systeln 2,856,240 10/1958 Breazczle et al. ..308/1O 2,981,111 4/1961Mcllwraith et al ..73/453 PATENTEDLFR 1 01973 Fig.1

Power Oscillator I S pp y Phase Dei ector Display Owen Orlando FietINVEN'IUR,

ATTORNEY DENSITY MEASURING INSTRUMENT HAVING ELECTROMAGNETIC SUSPENSIONWITH A VAABIJE SPRING CONSTANT The invention described herein was madein the perforrnance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION The present invention relates in general tolevitation devices and more specifically to density measuringinstruments which sense buoyant forces on a buoyant member suspended byelectromagnetic means.

It is well known that the force acting on a body immersed in a fluid isa function of the density of the fluid and of the volume of fluiddisplaced by the body. Thus, if the volume of the body is maintainedconstant and if the buoyant forces acting upon the body can be measuredthen the density of the fluid may be determined. Many density measuringsystems have been devised in which a body having magnetic properties issuspended within a magnetic field produced by electromagnets. Suchsystems often include an electrical feedback circuit and a positionsensing network which cooperate to maintain the buoyant member at a nullposition while under the influence of buoyant forces and in which anapplied voltage or current is representative of the buoyant forces andthus the density of the fluid in question. Such a system may be found inthe US. Pat.

to Mcllwraith et al., No. 2,981,111. A system such as shown thereinutilizes direct current differential electromagnetic suspension whichrequires that the suspended element have magnetic properties. Allavailable magnetic properties have magnetic hysteresis which could causehysteresis errors in the indicated force readout. Errors of thismagnitude may often be ignored, however, in systems which are used undervacuum conditions or for laboratory instruments which are to be used asstandard reference devices it is desirable to eliminate or minimize asmany sources of error as possible. The hysteresis error can be greatlyreduced or avoided by eliminating the use of magnetic material in thesuspended or buoyant element. An induction repulsion electromagneticsuspension system can avoid the use of suspended magnetic materials andof solid material connections to the suspended element.

The principles of electromagnetic levitation by eddy currents haveheretofore been used to suspend an electrical conductor in the magneticfield produced by an alternating current in an electrical coil. In suchcases it has been common to suspend the element either above or belowthe coil and in some cases to add an extra coil to obtain lateralstability of the suspended element. The stabilizing coil is connected inseries with the suspending coil. One of the few applications of thismethod of suspension is the radio-frequency levitation melting ofmetals. The speciman of metal is both melted and maintained insuspension to eliminate contamination of the melt by contact withcrucibles. Such a system is described in the US. Pat. to Wroughton, etal., No. 2,686,864 (1954).

The principles of levitation have also found application in maintaininga floated gyroscope centered within a frame as discussed in the US. Pat.to Anschutz- Kaempfe, No. 1,589,039 (1926).

It is accordingly an object of the present invention to provide adensity measuring instrument which is not subject to these and otherdisadvantages and limitations of the prior art.

It is another object of the present invention to provide a densitymeasuring instrument capable of measuring the density of gases which areat partial vacuum or substantially total vacuum pressures.

A further object of the present invention is to provide a densitymeasuring instrument which, in part, immerses a buoyant member in thefluidwhose density is to be measured and in which the buoyant memberdoes not include magnetic materials.

It is yet another object of the present invention to provide a densitymeasuring instrument in which the effective spring constant of thesystem may be varied.

SUMMARY OF THE INVENTION The present invention provides a device formeasuring the density of a fluid either liquid or gas. The device useselectromagnetic levitation by means of eddy currents to suspend anelectrically conducting element within an electrical coil which carriesalternating current. A housing having high magnetic permeability servesto concentrate a magnetic field around the suspended element and mayalso serve as a housing for confining the fluid whose density is to bemeasured.

A position sensing means is provided to determine the location of thesuspended element along the vertical axis and to provide an electricalsignal representative of the deviation of the suspended element frompreselected position referred to as a null position. An optical systemor a bridge circuit will serve this purpose. The deviation signal thenis incorporated into a feedback circuit to regulate the alternatingcurrent supplied to the suspension coil so as to maintain the suspendedelement substantially at the null position. The current or voltage isthen representative of the density of the fluid in which the suspendedelement is immersed. The accuracy of the system will be enhanced if thefeedback circuit utilizes a phase lock loop.

An opening is provided in the upper wall of the housing therebydecreasing the permeability of the flux path in the region above thesuspended element. If the space between the suspended element and thehousing is relatively small, the opening in the top wall of the housingcan be made sufficiently large so as to make the net repulsive forces onthe suspended element to act in an upward direction. The effectivespring constant of the system will also be affected thereby.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagram in cross-sectionrepresenting a density measuring instrument in accordance with theprinciples of the present invention;

FIG. 2 is a graphical representation of the electromagnetic forcesacting on the suspended element;

FIG. 3 is a graphical representation of the force gradient taken alongthe vertical axis of the suspended element; and

FIG. 4 is a diagram representing an electrical circuit suitable for thepractice of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I there isdepicted a suspended element 10 or buoyant member which is shown in theform of a hollow thin walled sphere and which is constructed of anynon-magnetic electrically conducting material such as aluminum. Forpurposes of discussion there is also shown a vertical axis 11 and ahorizontal axis 12 which intersect at the midpoint of the physicalsystem and which may coincide with the null position referred to below.FIG. 1 also shows an electrical suspension coil 13 substantially in theshape of a cylinder and having its axis coinciding with the verticalaxis 11. The sphere 10 and the suspension coil 13 are enclosed within amagnetic housing 14 or core which is substantially in the shape of aclosed end cylinder having end walls 15 and 16 and may be of anymaterial having high magnetic permeability, and high electricalresistivity.

If an alternating current is carried by the suspension coil 13 it isclear that eddy currents will be induced in the conductive member 10. Itis also apparent that the current carrying suspension coil 13 will havea magnetic field associated therewith and that the eddy currents in thesuspended element 10 will also have a magnetic field associated with it.These two magnetic fields are always in opposition to one another andthus there result repulsive forces between the two fields. If thesuspended element 10 is positioned exactly at the mid point 17 of thesuspension coil as defined by the intersection of the vertical axis 11and horizontal axis 12, then there will be forces acting downward on theupper portion of the sphere l and forces acting upward on the lowerportion of the sphere l0 and the sphere would be in exact equilibrium.

This condition is represented in FIG. 2 wherein the forces F acting onthe sphere are plotted along the vertical axis 11 and are represented bythe curve 18. As heretofore indicated there is a symmetry of forcesabove and below the horizontal axis 12 and that those forces are inexact opposition. The gradient of these forces along the vertical axisis shown in FIG. 3 wherein the vertical axis 11 is further representedby the letter Z. It is noted that the gradient dF/dZ is zero at the midpoint 17 and has symmetry above and below that point.

The system as thus far described is unstable in that if the sphere 10 isdisturbed from its point of equilibrium the forces acting on the spherewill have a resultant force in the direction of the disturbance. Upwarddisturbing forces will be present from buoyant forces and downwarddisturbing forces are present from the effects of gravity. To overcomethe foregoing, and as shown in FIG. 1, there is an opening 20 providedin the upper wall 16 of the housing 14. This opening serves to disruptthe high permeability path provided by the housing 14 and to disrupt theflux pattern of the magnetic field produced by the alternating currentand thereby redistribute the forces acting upon the sphere 10. Theredistribution of forces is represented in FIG. 2 by the curve 21 whichis intended merely to show the relative changes in forces and is notintended to be shown to scale. Since the flux field has been weakenedabove the sphere, the forces acting on the lower portion of the spherewill be the strongest and the maximum force acting on the sphere willoccur somewhat below the horizontal axis 12 and the net or resultantrepulsive force will be upward. As shown in FIG. 3, the zero gradientpoint has now moved somewhat below the horizontal axis 12 to a new pointsuch as indicated at 22. If there is no current in the suspension coil13 of FIG. 1 the sphere 10 will of course drop and rest against thebottom wall 15 of the housing 14. The center of the sphere will then beat some point below the horizontal axis 12 and it is necessary thatunder those conditions the center of the sphere be located at some pointabove the zero gradient position 22 shown in FIG. 3.

Under these conditions, when an alternating current is passed throughthe suspension coil 13 there will always be a resulted upward forceacting upon the sphere. Thus, if the sphere drops below the horizontalaxis 12 the suspension coil current may be increased to return thesphere to the mid point or null position and if buoyant forces cause thesphere to rise above the horizontal axis 12 the suspension coil currentmay be decreased to thereby return the sphere to the mid point. Themagnetic housing 14 is preferably constructed with rotational symmetryabout the coil axis, but may be constructed of segments rather than ofcontinuous construction as shown in the drawings.

Since the instrument may be used to measure the density of a gas as wellas a liquid, there is a cover plate 23 provided to seal the opening 20in the upper wall 16 of housing 14. This cover plate may be of any nonmagnetic material and may include a passageway 19 throughwhich the fluidwhose density is to be analyzed may be admitted to the interior chamberof the housing 14. In many applications such as those involving vacuumconditions, the instrument absent the cover may be placed within avacuum chamber.

In order to provide a mechanism for determining the position of thesphere 10 with respect to the horizontal axis 12 there is included inthe instrument an upper electrical coil 24 and a lower electrical coil25. These two coils are shown in FIG. 4 along with the remainder of theelectrical feedback network. Each of the coils 24, 25 form a leg of aninductance bridge network which also includes coil 26. The bridge isexcited through the transformer action of core 27 and primary coil 28.The output of the bridge is taken across the output lead 30 and groundand the output signal will vary as a function of the position of thesphere 10 between the upper and lower coils.

In order to minimize the influence of electrical noise on the accuracyof the system, the bridge output signal is impressed upon a filternetwork 31 which passes a very narrow band of selected frequenciesproviding an output which is impressed upon a high gain amplifier 32.The filter is selected in accordance with criteria well known in the artto satisfy stability requirements of a closed loop control system aswell as to minimize noise modulation within the system. For the purposesof detecting very small buoyant forces, it is desirable to have minimalrestoring and suspension forces acting upon the sphere l0 and at thesame time to have only small disturbances in the position of the sphere10 result in immediate restoration of the sphere to its null position.Thus, it is desirable to have a high gain amplifier, preferably having again in the order of 1 million to one.

As also shown in FIG. 4 the output of amplifier 32 is impressed upon aphase detector 33 which compares the phase relationship between theamplifier output signal and the alternating current bridge signalproduced by an oscillator 34. The output of phase detector 33 isimpressed upon the oscillator 34 which is used within the oscillatorcircuitry to continually adjust the oscillator output frequency to thepreselected value which will be efficiently passed by the filter network31. In other words, since the oscillator frequency and the band passcharacteristics of the filter may incur some small drift from the designpoint, it is desirable, and necessary for a very narrow band pass, tohave the oscillator frequency continually adjusted to remain within theselected band. The phase detector output amplitude is used to regulate apower amplifier 35 which in turn supplies alternating current to thesuspension coil 13. Thus, the alternating current may be continuouslyregulated and varied to always return the sphere to its preselected nullposition. The output amplitude of phase detector 33 is also indicativeof the alternating current level required to maintain the sphere 10 atits null position and is thus impressed upon display unit 36 which maybe devised and calibrated to provide a visual and/or printed record ofthe fluid density.

A variation of the foregoing instrument is included in FIG. 1 whereinthe upper wall 16 of the housing 14 includes a threaded insert 39 inwhich the opening 20 is provided. Several inserts each having an openingof a diameter differing from one another provide an easy means forvarying the opening in the upper wall to thereby vary the springconstant of the system. Other such means may be devised to serve thispurpose such as an iris type adjustment.

It will be apparent to those in the art that various techniques may beemployed to sense the position of the sphere 10 including optic systemsin which the sphere interrupts or reflects varying amounts of light toprovide a deviation signal representative of the sphere position. Alsoapplicable are inductance, resistive, or capacitance bridge networks, orcombination thereof.

The instrument as described and claimed lends itself to ruggedconstruction and is thus suitable for remote density sensingapplications such as required in the field of oceanography. Theprinciples of the invention also are applicable to a variety of powersupply conditions including the commonly found 60 cycle supply and to400 cycle supplies found in some fields of applications.

What is claimed is:

1. An instrument for measuring density of a fluid comprising:

a. a vertically aligned cylindrical electrical suspension coil;

b. a magnetic core enclosing said coil and having a single openinglocated in the upper wall thereof; an electrically conducting buoyantmember to be immersed in the fluid and positioned within said coil andsaid core, the opening in the upper wall of said magnetic core being inthe region axially above said buoyant member and selectively sized so asto provide upwardly directed expulsive forces on said buoyant member forall positions of said buoyant member within said core and coil wheneveralternating current is applied to said coil;

d. sensing means coupled to said buoyant member for detecting adeviation of said member from a null position and for producing adeviation signal representative thereof; and

. a feedback circuit responsive to the deviation signal and coupled tosaid coil for producing an alternative coil current which tends toreturn said buoyant member toward its null position whereby the coilcurrent or applied voltage is representative of the buoyant force onsaid member.

2. The instrument of claim 1 wherein said buoyant member is sphericallyshaped.

3. The instrument of claim 1 wherein said buoyant member is hollow.

4. The instrument of claim 1 wherein said buoyant member is a hollowsphere.

5. The instrument of claim 1 further comprising means associated withsaid magnetic core for varying the size of the opening therein.

6. The instrument of claim I wherein said feedback circuit includes:

a. a narrow band filter having the deviation signal impressed thereon;

a high gain amplifier having its input coupled to the output of saidfilter; and

a power amplifier for supplying alternating current having its inputcoupled to the output of said high gain amplifier and the output coupledto said suspension coil and providing current to said suspension coil inresponse to the deviation signal.

The instrument of claim 6 further including:

. an electrically controlled oscillator having its output coupled tosaid sensing means; and

. a phase detector coupled between said high gain amplifier and saidpower amplifier and further coupled to said oscillator, said detectorincluding means for comparing the phase difference between the high gainamplifier output signal and the oscillator output signal and forproviding a phase signal representative thereof, said oscillator beingresponsive to the phase signal to maintain the oscillator outputfrequency within the frequency band pass of said filter.

1. An instrument for measuring density of a fluid comprising: a. a vertically aligned cylindrical electrical suspension coil; b. a magnetic core enclosing said coil and having a single opening located in the upper wall thereof; c. an electrically conducting buoyant member to be immersed in the fluid and positioned within said coil and said core, the opening in the upper wall of said magnetic core being in the region axially above said buoyant member and selectively sized so as to provide upwardly directed expulsive forces on said buoyant member for all positions of said buoyant member within said core and coil whenever alternating current is applied to said coil; d. sensing means coupled to said buoyant member for detecting a deviation of said member from a null position and for producing a deviation signal representative thereof; and e. a feedback circuit responsive to the deviation signal and coupled to said coil for producing an alternative coil current which tends to return said buoyant member toward its null position whereby the coil current or applied voltage is representative of the buoyant force on said member.
 2. The instrument of claim 1 wherein said buoyant member is spherically shaped.
 3. The instrument of claim 1 wherein said buoyant member is hollow.
 4. The instrument of claim 1 wherein said buoyant member is a hollow sphere.
 5. The instrument of claim 1 further comprising means associated with said magnetic core for varying the size of the opening therein.
 6. The instrument of claim 1 wherein said feedback circuit includes: a. a narrow band filter having the deviation signal impressed thereon; b. a high gain amplifier having its input coupled to the output of said filter; and c. a power amplifier for supplying alternating current having its input coupled to the output of said high gain amplifier and the output coupled to said suspension coil and providing current to said suspension coil in response to the deviation signal.
 7. The instrument of claim 6 further including: a. an electrically controlled oscillator having its output coupled to said sensing means; and b. a phase detector coupled between said high gain amplifier and said power amplifier and further coupled to said oscillator, said detector including means for comparing the phase difference between the high gain amplifier output signal and the oscillator output signal and for providing a phase signal representative thereof, said oscillator being responsive to the phase signal to maintain the oscillator output frequency within the frequency band pass of said filter. 